1. Field of the Invention
The present invention relates to a process for producing oil-containing microcapsules, and more particularly to a process for microencapsulating an oily liquid by coacervation utilizing gelatin as at least one of the hydrophilic colloids, whereby oil-containing microcapsules, whose walls are less porous and thicker, can be obtained without aggregation.
2. Description of the Prior Art
The process of producing microcapsules by coacervation has hitherto been described in U.S. Pat. Nos. 2,800,457 and 2,800,458, and is classified into simple coacervation or complex coacervation depending upon the liquid-liquid phase separation of the colloid of the hydrophilic high molecular weight electrolyte.
The term, "liquid-liquid phase separation" is intended to cover such an operation that a solution of one or more kinds of high molecular weight electrolyte colloids is separated into two different liquid phases, i.e. a colloid-rich phase and a colloid-poor phase, and such phase separation phenomenon is usually termed "coacervation". The colloid-rich phase which is obtained by the phase separation is called the "coacervate", and when the thus obtained coacervate is composed of a single kind of colloid, the operation is called simple coacervation and, when composed of two or more kinds of colloids, the operation is called complex coacervation.
Microcapsules have the following advantages:
1. the liquid is outwardly in the form of a solid, PA1 2. two or more kinds of reactive materials can be preserved for a long term in the form of a mixture because they are isolated with the wall, PA1 3. a liquid in which a solid material may be dissolved has the capability of protecting the material included therein from the environment and stored for a desirable term, PA1 4. color, taste or toxicity can be intercepted, PA1 5. the release of liquid in which a solid substance may be dissolved can be controlled, PA1 6. the liquid can be maintained on a plane, and PA1 7. handling of the liquid is easy. PA1 a. Since the formation of capsules can be achieved in a higher colloid concentration, a large quantity of the capsules can be produced and therefore, production costs can be reduced, and the amount of coater to be evaporated is small, whereby thermal means and apparatus therefor are more simple. PA1 b. A liquid to be encapsulated is effective to the amount of colloid. PA1 c. All steps of the above process are continuously conducted because the elevation of the viscosity is not caused and the step for hardening the system does not become the velocity controlling step, and the load for the stirrer is reduced. PA1 1. dispersing a hydrophobic fine powder or emulsifying a hydrophobic liquid into an aqueous solution of at least one kind of high molecular weight electrolytic colloid which is used for forming the capsule wall (dispersing or emulsifying step); PA1 2. subjecting the dispersion or the emulsion obtained in step (1) to water dilution, and then salt addition and/or pH adjustment thereof. In this case, an aqueous solution of a high molecular weight colloid may be added thereto, if desired (coacervation step); PA1 3. cooling the formed coacervate to gell it (cooling step); PA1 4. adding a phenolic compound thereto at a temperature of above 5.degree.C, preferably above 8.degree., during the steps of (2) and (3); PA1 5. adding a hardening agent to the system; PA1 6. adding a shock-preventing agent at a temperature lower than the gelling point of gelatin; wherein by "shock" is meant such a phenomenon that the viscosity of the system elevates rapidly, i.e., at a pH not less than near (about) 6, generally at a pH not less than near 6 to 13 and not commonly at a pH of not less than near 6 to 9, when conducting the pre-hardening of the coacervation capsule liquid containing the gelatin, and by "shock-preventing agent" is meant a solution for prevening the shock; PA1 7. adjusting the pH of the system to the alkali side (the foregoing steps (5), (6) and (7) (pre-hardening treatment); and PA1 8. optionally elevating the temperature of the system to more effectively harden the coacervate (hardening step). PA1 pectinic acid - polygalacturonic acid in the colloid form containing some methyl ester groups PA1 pectin - water soluble pectinic acid containing methyl ester groups.
Therefore, applications thereof to agricultural chemicals, chemicals, medicine, perfumery, adhesives, liquid crystal, paint, foods, detergents, dyes, solvents, catalysts, enzymes, anticorrosives and the like have been studied. Especially, aspirin-containing capsules, perfumery-containing capsules, menthol tobacco, pressure-sensitive adhesive capsules, reaction type adhesive capsules, color-former containing capsules for pressure-sensitive recording paper, anticorrosive-containing capsules for rivets are manufactured industrially.
While various microencapsulating methods have been proposed for these purposes, the coacervation process is the most useful. The process of forming microcapsules according to coacervation is suitable for microencapsulating a liquid droplet itself or a liquid droplet containing a material dissolved or dispersed therein, but controlling the various conditions therefor is difficult. Namely, the formation of the coacervate according to a simple coacervation is influenced by the colloid concentration, salt concentration, proportion of colloid to the material to be microencapsulated, the stirring state of the system, the temperature of the system and the like; and the formation of the complex coacervate is influenced by the colloid concentration, the colloid ratio, the pH of the system, the temperature of the system, the proportion between the colloid and the material to be microencapsulated, the stirring state of the system, properties of the colloids used, and the like. In these conditions for coacervations, there are, of course, optimum conditions.
Accordingly, as the system is slipped out from the optimum conditions, the coacervation is insufficiently carried out and less of the coacervate is produced. For example, when the concentration of the colloid is low, the coacervation is sufficiently carried out whereby the production of coacervate is increased, and if the concentration of colloid is high, the coacervation is insufficiently carried out and less of the coacervate is produced.
Since gelatin is used as one of the colloids for improving the thermal properties of the wall (for example, the wall is not melted at a temperature higher than 70.degree.C) and a hardening agent such as formaldehyde is used for hardening gelatin, it is further difficult to carry out the process.
Such formation of microcapsules by the utilization of the coacervation process has various demerits such as (a) complicated establishment of the conditions for forming the coacervate, (b) necessity of gelatin as at least one kind of wall membrane-forming colloid, (c) requirement of treatment for imparting a high heat-resistance to the wall membrane and (d) undesirable limitations in the process and in the shape of the formed microcapsule.
Typical limitations in the process are based on the following items:
An operation of concentrating the capsules is required because the colloid concentration for producing the capsule is limited to 2.5% (a main reason of limiting the concentration is depending upon preventing aggregation of the capsules owing to the elevation of viscosity in the system when rapidly converting the pH to harden the coacervate), there is a large difference in the slurry viscosity because gelatin is not deposited around the liquid to be encapsulated and remained in the system, and to hardening the wall of gelatin, pH is not rapidly converted to alkalinity.
The limitation on the shape of the formed microcapsules is based on the following items:
Grape-like capsules (hereinafter, referred to as "poly nucleus capsule"), which are numerous aggregated droplets, are produced due to the limitation of the colloid concentration, a porous wall is formed and the strength of the wall becomes the same as that property of the gelatin because most of the negative electrolytic colloid having an electric charge opposite to the gelatin is pulled out into the system equilibrium solution at the step of completing formation of the capsule, and the viscosity of the slurry is increased and the thus formed microcapsules are aggregated in the step of changing the pH to the alkali side because the microcapsules are formed with high concentration of colloid and hardened.
An important aspect of the present invention is to improve the foregoing limitations, to rapidly adjust the pH of the slurry to the alkali side in the presence of a hardening agent for preventing elevation of the viscosity, stabilizing and hardening the wall at the step of making the capsule in a higher colloid concentration, to control proper sizes of microcapsules containing one liquid droplet (hereinafter, referred to as "mono nucleus capsule") or polynucleus capsules and to reduce the porosity of the wall thus prepared.
That is to say, the process of the present invention provides a practical multi-useful process on an industrial scale.